The present embodiments relate to an electron source and a method for the operation of an electron source.
An electron source that may be used in an X-ray tube of an imaging medical engineering device is, for example, known from DE 10 2007 042 108 B4. The electron source includes electron emission cathodes and a plurality of control electrodes. An electrically insulating data transmission link (e.g., an optical data transmission link) is provided for data transmission between a high-voltage unit provided for supplying energy to the electron source, and a low-voltage unit.
Electron sources in multifocus X-ray tubes with control electrodes constructed, for example, as grids may work with field emitters such as carbon nanotube (CNT)-emitters based on CNTs or thermal emitters. The principle of an electron source with CNTs is known, for example, from DE 10 2009 003 673 A1.
The emission of electrons is determined by an electrical filed strength on a surface of the electron emission cathode and may be set by a voltage applied at a grid-like control electrode (e.g., a control grid). The relationship between the voltage and the generated electron current may be described by an exponential characteristic curve. Over the lifetime of an electron source, the exponential characteristic curve is subject to changes. The changes in the exponential characteristic curve have origins, for example, in damage to and/or aging of the electron emission cathodes and may be offset by a regulator that adjusts the control voltage (e.g., the voltage applied between the electron emission cathodes currently in operation and the control grid).
In an X-ray tube, the electrons emitted by the electron emission cathodes are accelerated to an energy necessary to generate the X-ray radiation by the high voltage applied to the anode of the X-ray tube. The electrons arriving at the anode define the tube or anode current. This depends on, among other factors, a geometric arrangement of individual components within the X-ray tube, on the control voltage, and on numerous other influencing values (e.g., temperatures of components of the electron source such as a temperature of the electron emission cathodes, an on-time of the X-ray tube, the cathode current and a vacuum level within the X-ray tube). In addition, the operation of the X-ray tube to date (e.g., a history of the X-ray tube) may influence a dose-determining tube stream. For example, the control grid used and an operating state of the control grid influences the tube stream.
The anode current determining the X-ray dose may be produced from the cathode current minus a current flowing out via the control electrode. The relationship of anode current to cathode current is defined as a transmission rate and may be determined, for example, with the aid of a learning procedure. The transmission rate determined may be assumed to be constant or at least only slowly changeable. Measurement of the cathode current is thus suitable for determining a generated dose of X-ray radiation. This measurement may, for example, take place via a measuring resistor. As a result of capacitive loads in the measurement arrangement implemented in the control electronics of the X-ray tube, limitations of this measurement principle do, however, exist in the case of rapid switching procedures.
In order to determine the dose of generated X-ray radiation based on the release of electrons by the electron emission cathodes, two interrelationships are thus, for example, to be considered: the characteristic curve of the electron source and the transmission rate of the X-ray tube.
An electron source may be regulated by a voltage regulation. The current/voltage characteristic curve of each electron emission cathode may be determined with the aid of a learning procedure. Current values assigned to the voltage values are stored in a table for each of the individual electron emission cathodes. The tables, which represent characteristic curves of the electron emission cathodes, remain unchanged. Thus, aging or drifting of electron emission cathodes is also ignored, as is the case with changes in the transmission rate.
Given sufficiently long pulses (e.g., from around 1 ms), the aging and drifting may be offset by an overlaid current regulation, in that the set value specified is readjusted for the voltage. However, in the course of the readjustment, charge transfers that adversely affect the cathode current take place. In the case of small anode currents, capacitive currents have an effect on the measurement in a relevant manner.
Capacitive charge transfer effects, for example, restrict the applicability of the overlaid current regulation in the case of short pulses. As a result of these charge transfer effects, an estimation of the anode current may only be possible after approximately 40 μs on the cathode side. A prerequisite for a readjustment is thus a significantly longer pulse duration. A direct measurement and regulation of the dose-determining anode current, which may come into consideration instead of a cathode side measurement and regulation, is, however, not possible with pulse durations of the aforementioned order of magnitude according to the prior art. Measurement of the anode currents may thus take place in the generator at low potential at a generator low end. Strong low pass filtering is used in order to avoid disturbing variables. Time constants thus supplied may lie in the order of 70 μs, which corresponds to an order of magnitude of a desired, short pulse duration.